He will and can rurn back and forth between old and young form if he turn that sandglass upside down in his dreams. His next transformation into a younger form is just a few chapters away
The good thing about this story is that the couple's mental age stays 'old' even if their body changed....
not like most 'old guy became younger' stories.... granted that 60/70+ is not the same like 30/40+, but the point still stand.